Current limiting device for lenard tubes



April 3, 1934. F. EHRKE El AL 1,953,470

CURRENT LIMITING DEVICE FOR LENARD TUBES Filed Feb. 14, 1930 INVENTORS 77CMSL L..F'.EH7EKE ATTORNEY Patented Apr. 3, 1934 iCE CURRENT LIMETKNQ: DEVICE FOR LENARD TUBES Lou s Frederick Ehrhe. Newark, and Charles Morse Slash, Bloomfield, N. 3., designer to Westinghouse Lamp Company, a corporation of Pennsylvania Application February 14, 1930, Serial No. 428,271

2 Claims.

The present invention relates to Lenard ray tubes and more particularly to current limiting means embodied in such tubes to protect the window thereof.

The inherent weakness of the Lenard ray tube lies in the fact'that a single surge of high current may destroy the window.

Such destruction of a window necessitates near ly complete rebuilding of the tube since the open ations such as, evacuating the tube, degasifying of metal parts and removing impurities have to be repeated in addition to the replacement of the window itself. The windows are extremely thin and diflicult to construct and mount in the tube. In addition any shielding means such as a metallic coating deposited on the window has to be reformed.

An object of our invention is to provide an improved Lenard ray tube in which a space charge at the cathode will limit the electron current to such a value that the window will not be injured regardless of the filament'temperature.

A further object of our invention is to provide a Lenard ray tube in which the cathode shall be so shielded that the electron current will be limited to a safe value for a wide voltage range.

In carrying out our invention we provide a shielding means disposed about the cathode in such a manner and at a potential such that the electron current of the tube will be limited to a value such that it cannot injure the window.

For a better understanding of our invention reference may be had to the accompanying drawiiig in which,

Fig. l is a central sectional view of a Lenard ray tube embodying our invention, and

Figs. 2 and 3 are fragmentary sectional views of other embodiments of our invention which may be substituted for the corresponding parts to similar parts A 9 are sealed in a press 10 at the outer end of said reentrant stem 8.

The inner end 11 of stem 8 is closed thus providing a space 12 between the walls of the stem which space is sealed from the interior of the envelope 5 and open to the exte rior thereof. The inner surface of stem 8 which borders the space 12 may be provided with a coating 13 of metal such as gold, silver or aluminium in the form of a thin layer deposited thereon. Tin foil or other suitable conductive material may also be used for the metallic coating 13. One of the cathode leading in conductors 9 is electrically connected to the metallic coating 13 at point 14.

In many instances the metallic coating 13 may be omitted from the stem 8 since the cathode leading-in conductors 9 will increase the ionization of the air in the space 12 and render the same conducting. The column of air in space 12 wiil then function as a conductor and indlwe a charge of the same potential as that of the leading-in conductors 9 to the walls of the stem 8 adjacent the cathode to shield the same. The metallic coating on the stem 8, however, appears to render the operation of the tube more constant. At the end of envelope 5 opposite the reentrant stem 8 a neck 18 is formed in which the hollow anode 6 may be supported by a suitable split sleeve 17. A press 18 is formed in the neck 15 at a point adjacent the side of the hollow anode 6 and leadingdn conductor 19 for said anode 6 is sealed in this press.

At the outer end of the neck 16 a window 20 perinc-able to Lenard rays is constructed in accord ance with the disclosure of Patent 1,735,302 assigned to the same assignee as the present invention.

Briefly this window 20 comprises a bulbous body 21 of vitreous material of relatively heavy construction having a concentric reentrant portion 22, said reentrant portion 22 having at the center thereof a thickness of from about 0.0001 to 0.005 inches.

The reentrant stem 8, the hollow anode 6 and the window 20, are disposed in alignment along the central longitudinal axis of the envelope 5, so that when a low potential is impressed on the cathode leads 9 to bring the cathode '7 to incandescence and a high potential is impressed on the cathode and anode, electrons will be emitted by cathode 7 and directed through the hollow anode 6 and window 20.

According to the present invention the reentrant stein 8 together with its metallic coating 13 serves a three-fold purpose because of the electrical connection between the coating 13 and one of the cathode leading-in conductors 9, and the disposal of the cathode 7 within said reentrant stem. First, said reentrant stem and coating focus the stream of electrons to a suitable sized spot on the window or a target beyond the window; second, they shield said cathode so that electrical discharges originating at the cathode and tending to puncture the envelope when high voltage is applied will not occur and third, by virtue of their arrangement with respect to the cathode they limit the current to such a value that the window of the tube will at all times be protected. To this end the cathode '7 is withdrawn into the bore of the reentrant tube a distance approximately three and one half times the diameter of the bore of said reentrant stem.

In this construction the space charge at the cathode 7 afiorded by the coating 13 which is at the same potential as the cathode will limit the electron current to a value which does not injure the window regardless of the filament temperature.

For example, we have found that by placing the filament at a depth of about 2 cm. in a focusing cup '7 mms. in diameter that the electron current was limited to approximately 0.1 a reasonable value for the window.

A similar result can be obtained by placing a diaphragm of the correct size: over the'mouth of focusing cups now in general use. Such a construction is illustrated in Fig. 2 in which 25 is a focusing cup and 26 a filamentary cathode therein supported by leadingdn conductors 27. A press 28 at the end of a stem 30 seals in the leading-in conductors27. A sleeve 29 is secured to the outside of press 28 and its supporting stem 30 and extends inwardly to the mouthof focusing cup 25. A.m etallic ring 31 may be used to supplement the su fiflbrt of focusing cup 26 and electrically connect said focusing cup to the sleeve 29. Cathode 26 is electrically connected to the focusing cup 25 through a conductor 32 leading to one of the leading-in conductors 2']. A second sleeve 33 split through a part of its length by opposed slots 34 is mounted on the outside of sleeve 29 making a sliding fit therewith. A diaphragm 35 closes the inner end of sleeve 33 except for a central opening 36 therein.

(In this construction the ratio of the depth of the filament from the diaphragm to the diameter of the diaphragm opening 36 may be adjusted by tapping the ends of the tube since this adjusts the'positions of sleeves 29 and 33 relative to each other! In this way the limiting value of the electron'current may be adjusted as desired.

Referring to Fig. 3 a further embodiment of our invention is illustrated in which a fixed ratio between the filament depth and the focusing cup diameter is obtained to restrict the electron current to a fixed limiting value. In this embodiment a focusing cup 25 contains a cathode filament 26 disposed at a depth of about three and a'half' times the bore of the focusing cup. Leading-in conductors 27' and sleeve 29 support the cathode filament 26 through the intermediary of conductor 32 and metal ring 31 in the same manner as described relative to the correspond ing elements in the device shown in Fig. 2.

A press 28' on stem 30' forms a support for the sleeve 29 as before. Numeral 6 in Figs. 2 and 3 designates 'in broken lines a portion of the anode shown in full in Fig. 1. v

Ihe current limiting means according to our invention is applicable to a wide range of voltages impressed upon the tube. Although the current of the tube increases as the voltage increases the absorption of energy by the window affords a compensating factor. As disclosed in the Slack Patent 1,735,302, above referred to, the absorption of energy by the window of the tube varie s with the voltage, decreasing as 4 the voltage increases. Thus for instance with a glass window of about 0.00025 inch thickness, it requires a potential of about 26,000 volts between the anode and cathode to cause the most of the cathode rays to just get through the window. At this voltage all of the energy of the electron is absorbed by the glass which, as a consequence heats up. As the voltage applied between the electrodes increases,however, the loss of energy in the glass window decreases rapidly.

With a velocity of the electron corresponding to 00,000 volts the 0.00025 inch glass window will transmit electrons with an emergent velocity corresponding 130 154,000 volts, there being a loss of velocity in the window corresponding only 'to about 6,000 volts. Thus for instance, the same thickness of glass will absorb only about 31000 volts with an impressed potential 100,000 volts.

This decrease in'energy loss in the glass, at higher voltages, effects a corresponding decrease in the heating effect of the glass." It thus'becomes necessary to protect the window only against 0 heavy surges of current.

The invention has been described specifically in connection with a Lenard ray tube, but it is to be understood that the current limiting means is applicable to other devices and that the invention 0 is not limited to the exact construction shown but various modifications and changes may be made therein without departing from the invention as defined in the appended claims. 4

What is claimed is:

1. A Lenard ray tube comprising anenvelope, a source of electrons therein, a window in said envelope permeable to high velocity electrons, an anode for directing a stream of high velocity elecrons against said window, a reentrant stern in the form of a double walled hollow sleeve surrounding and extending beyond said "source" of electrons a distance ofapproximately three and a half times the bore ofsaid sleeve, said sleeve having a space open to the e'xterior'of the envelope and closed to the space therein, and ametallic shield dispos'ed'i'n the open space and electrically connected to 'saidsou'rce of electrons? 2. An electric discharge device comprising an envelope, a source of electrons therein, a window in said'envelope permeable to high velocitylectrons, an anode for' aireaingaspreairi of high velocity electrons against said window, a reentrant stem in the form of sequels walled hollow sleeve surrounding and extending beyondls aid 35 source of electrons a distance approximately three and one half times the bore ofsaid' slee id sleeve having a space open to the exterior of the envelope and closed to the space tl er'ein.

140, LOUIS FREDERICK EHRKE. CHARLES SLACK? 

